EVOLUTION OF
HORSES

INTRODUCTION
The horse belongs to order Perissodactyla and family Equidae. The
evolution history of horses is well known because:
In North America the
entire evolutionary
sequence of the horse
is recorded in the
form of fossils within
some undisrupted
and uninterrupted
series of sedimentary
deposit ranging in age
from early Eocene to
Recent.
Because of grazing
habit, their
adaptation to life on
savannas and plains
and their tendency to
live in large herds,
fossils of horse are
preserved and found
in large number.
The evolution of
horses shows
orthogenesis.

Adaptation of modern horses:
• The horse develops elongated slender limbs with unguligrade posture.
• The horse has undergone extreme evolution resulting in suppression of all the digits
except the middle one(III digits) so that the horse becomes an one-toed animal
• To make the quick movement most effective, the III digits of the limbs become greatly
elongated and the metatarsal/metacarpal of the leg forms the powerful cannon
bone.
• For rapid running horses have adapted a set of spring ligaments lying on the posterior
surface of foot, connecting the cannon bone with sesamoid bone and the latter with
the phalanges of the hoof.

• Horses have also developed a flexible and elongated neck .The growth of neck has been coupled with
elongation of frontal portion of the head anterior to eyes, thus developing an elongated muzzle.
• Such a result of elongation of jaw is the formation of a gap between anterior and posterior set of teeth
which is called diastema.
• The incisors of horse become chisel shaped with very sharp edges. Canines are of no use and hence
highly suppressed
.
• Transformation of all premolars into molars and developing high crown hypsodont teeth which grow
continuously as they are eroded throughout the life of the horse.
• The teeth of the horse becomes self sharpening, self renewing and highly efficient grinding tool.

The basic ancestral characters
Fossil evidence indicate
that condylarths are
probable progenitors of the
perissodactyls.
Lower Eocene Hyracotherium
is most primitive horse
exhibiting a close morphologic
similarity with the North
American paleocene
condylarths, Tetraclaenodon,
specially as regards its
structure of limbs and cheek
teeth.
The cusps of
Tetraclaenodon were found
transformed into
transverse ridges or
lophids.
The changes from
Tetraclaenodon to
Hyracotherium points to a
shift in adaptation for
effective browsing and
quick running in response
to pressure from
contemporaneous
aggressive predators.

OUTLINE OF PHYLOGENY OF HORSE

Progressive trends of evolution
Increase of size with height
increasing from 12 inches
to above 72 inches
Lengthening of limbs with
gradual shifting of ankle
and wrist away from the
toe.
Lengthening of neck and
straightening and stiffening
of back producing a
streamline body.

Reduction of numbers of
toes in limbs making all
the limbs one toed.
Lengthening of portion
of head in front of eyes
forming a mosaic.
Development of high
crowned cheek teeth
from original low
crowned brachydont
type with consequent
deepening of frontal
part of skull and lower
jaw to accommodate
those teeth.
Molarisation of
premolars.

Widening of incisors
with very sharp
edges.
Development of
complex fold pattern
on enamel on the
upper surface of the
crown of cheek teeth.
Gradual increase of
complexity and size
of brain.

A few more points
The evolution of horse shows positive allometry and negative allometry.
The history of evolution of horses is still not completely explained.
Cause of extinction Equus at the end Pliestocene in North America, inspite of
the change of climate in a favourable direction also remains unexplained.

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